What the A.400 alarm means

The main-circuit DC bus voltage exceeded the limit (about DC 420 V on 400 V class units); the SERVOPACK stops to protect the capacitors and power stage. This is one of the faults our repair team handles on Yaskawa DX100 / DX200 / YRC1000 systems — the cause is usually identifiable within the first on-site inspection.

Most common causes

  • Incoming AC supply voltage too high
  • Large regenerative energy during deceleration with a weak or missing braking resistor
  • Unstable plant power with voltage spikes
  • SERVOPACK detection circuit fault
Field note: the first item on this list is what we find most often on real machines. Check it before you go deeper.

Step-by-step field fix

  1. Measure incoming AC voltage and verify it is within the rated range
  2. Check the braking resistor and its wiring (an open resistor turns every decel into overvoltage)
  3. Look for voltage spikes from other heavy equipment on the same feeder
  4. Reduce deceleration rates and retry the motion
  5. Replace the SERVOPACK only after supply and regeneration paths are proven healthy
Safety first: never bypass safety circuits or force contactors closed. If the alarm involves E-stop chains, brakes or drive power, and you are not a trained robot technician, contact a professional — wrong moves here can cost you a robot or an injury.

Still stuck? Get an engineer on the phone — right now

Tell us the alarm code, the robot model and what was happening when the alarm came up. Our engineers run these diagnostics every day, and remote diagnosis by phone or video call is free.

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Hotline: +86-20-39029800 · WhatsApp: +86-188-1886-2860

Related Drive & Power Stage alarms

A.100 — Overcurrent or Heat Sink Overheated (SERVOPACK) A.320 — Regenerative Overload A.410 — Undervoltage — Main Circuit DC Voltage Too Low A.710 — Overload — High Load (Stall-level Torque) A.720 — Overload — Low Load (Continuous Over-torque) A.7A0 — Heat Sink Overheated (>100 degC)
Other brands: we also keep field-fix libraries for Universal Robots · Kawasaki · Staubli and more — see the complete alarm code hub.

Frequently asked questions

QWhat does the Yaskawa A.400 alarm mean?

The main-circuit DC bus voltage exceeded the limit (about DC 420 V on 400 V class units); the SERVOPACK stops to protect the capacitors and power stage. In field practice it belongs to the drive & power stage family of faults.

QCan I clear a Yaskawa A.400 alarm myself?

Sometimes. Work through the step-by-step checks on this page — the first listed cause is what we find most often in the field. If the alarm returns after two attempts, or the fault involves power, brakes or safety circuits, stop and call a qualified robot service engineer.

QHow fast can a Henghuan engineer help?

On-site response in the Pearl River Delta within 30 minutes to 3 hours, across mainland China in 1-3 business days, and worldwide within 2-5 business days. Remote diagnosis by phone/WhatsApp/video is available immediately.

Written by the Henghuan Robotics repair team from field service experience. Alarm descriptions are our own summary for troubleshooting reference and may vary by controller and software version — when in doubt, check your Yaskawa documentation or contact us. Henghuan is an independent service provider, not affiliated with or endorsed by Yaskawa.